The present invention relates to a fuel injection system for an internal combustion engine; and, more particularly, the invention relates to a technology for controlling fuel to prevent it from attaching onto a wall surface of an intake manifold by controlling the projection and penetration of the fuel spray (fuel spray travel distance).
A known conventional fuel injection system is disclosed in Japanese Patent Application Laid-Open No. 5-126012. In this publication, it is described that, since the speed of rotation of an engine is low and the closing time of the intake valve is relatively long when the engine is operated in a low speed range, such as during starting operation or idling operation, the fuel spray from a fuel injector of the electromagnetic type is preferably suspended inside the intake manifold, and the penetration force of the fuel spray is preferably weak and the fuel becomes fine liquid droplets.
It is also described in the above-referenced publication that, on the other hand, since the speed of rotation of the engine is high and the closing time of the intake valve is relatively short as the speed of rotation of the engine is brought from a middle speed range to a high speed range, the fuel spray must be rapidly supplied to the combustion chamber without becoming attached onto the inner wall surface of the intake manifold, and, accordingly, the penetration force of the spray needs to be strong.
In the system described in the above-referenced publication, the electronic fuel injection system of a multi-point injection type for controlling an internal combustion engine has an electromagnetic fuel injector for injecting fuel toward a rear dish-shaped portion of the intake valve for each cylinder. In this system, the shape of the fuel spray is varied depending on the operating condition of the engine by detecting the operating condition of the engine and by driving a fuel spray shape varying means of the electromagnetic fuel injector using a signal representing the detected operating condition.
The system varies the fuel spray shape by controlling the ratio of a swirl fuel component to a non-swirl fuel component in the injected fuel spray. The swirl force of the swirl fuel component is added when the fuel passes through a fuel passage (groove) for introducing an eccentrically to the direction of fuel flow along an axis, the fuel passage being formed in a fuel swirling element.
On the other hand, the non-swirl component is represented by fuel passing through a gap between an inner peripheral surface of the fuel swirling element and a ball which represents the valve body. For example, when the size of the gap is increased, the amount of fuel leaking through the gap is increased to increase the non-swirl component, and the fuel spray takes on a shape having a small injection angle, which is suitable for high speed operation of the engine.
In the system described above, a piezoelectric element is used to vary the size of the gap between the inner peripheral surface of the fuel swirling element and the ball which represents the valve body. Therefore, the fuel spray shape varying means using the piezoelectric element needs to be formed in a very narrow space of a nozzle end portion of the fuel injector, which causes a problem of improving the productivity.
Further, it is necessary to take the wiring to the piezoelectric element into consideration. In addition, the problem of reliability, such as a change in the characteristic and the durability of the piezoelectric element, should be considered.
In the above-mentioned publication, no consideration is given to a system for injecting fuel so as to coordinate the timing of fuel injection with the timing of the intake stroke. A lean burn engine is an engine in which a lean mixed gas is burned, and the mixed gas is made lean by employing a fuel injection method in which fuel is injected from a fuel injector provided in each cylinder (multi-point injection system: MPI) and by performing fuel injection in synchronism with the intake stroke.
In a fuel injection system required to perform fuel injection in synchronism with the intake stroke, for example, in an engine as described above, the time lag (transport lag) until the injected fuel spray reaches the inside of the cylinder becomes a problem. In a case where the penetration force of the fuel spray is varied, the time until the fuel spray reaches the inside of the cylinder is different between a fuel spray having a high penetration and a fuel spray having a low penetration. Therefore, it is necessary to take the transport lag of the fuel spray into consideration.
A first object of the present invention is to make it possible to control the penetration of a fuel spray without large modification of the fuel injector.
A second object of the present invention is to reduce the transport lag of the fuel spray even when the penetration of fuel spray is varied.
In order to attain the above first object, the swirl velocity component and the axial velocity component of the fuel are varied by controlling the rising or lifting speed of the valve body of the fuel injector. In detail, by increasing a speed of opening of the valve so as to rapidly increase the swirl force of the fuel, the swirl velocity component of the fuel is strengthened and the axial velocity component is weakened. By doing so, a fuel spray having a small penetration force (a low penetration fuel spray) can be obtained.
On the other hand, by decreasing the speed of opening of the valve, thereby gradually increasing the swirl force of the fuel, at the initial stage of the fuel injection, the swirl velocity component of the fuel is weakened and the axial velocity component is strengthened. By doing so, a fuel spray having a large penetration force (a high penetration fuel spray) can be obtained. Because the method does not require a modification of the structure and does not cause complexity in the constitution of internal parts, the method described above has the advantages of providing a very low cost and an improvement in the reliability compared to a method based on mechanical operation.
In order to attain the above second object, the intake air velocity is changed, corresponding to whether a high penetration fuel spray or a low penetration fuel spray is to be provided, by varying the cross-sectional area of the air intake passage in the intake manifold upstream of the fuel injector. In detail, in the case of a low penetration fuel spray in which the fuel spray can reach only a position upstream of the intake valve, the transport lag of the fuel spray is reduced by increasing the velocity of the intake air flow velocity.
Increasing the intake air flow velocity may be performed by decreasing the cross-sectional area of the intake air passage. In that case, the intake air flow in a bending passage of the intake manifold is guided on a passage wall side opposite to a position in the intake manifold where the fuel injector is arranged. Thereby, it is possible to prevent the fuel spray from attaching onto the passage wall on the opposite side of the position in the intake manifold where the fuel injector is arranged, or to reduce the amount of fuel spray attaching onto the passage wall.